Pharmacokinetic drug interactions with cyclosporin (Part II)

Precision sirolimus dosing in children: The potential for model-informed dosing and novel drug monitoring

The mTOR inhibitor sirolimus is prescribed to treat children with varying diseases, ranging from vascular anomalies to sporadic lymphangioleiomyomatosis to transplantation (solid organ or hematopoietic cell). Precision dosing of sirolimus using therapeutic drug monitoring (TDM) of sirolimus concentrations in whole blood drawn at the trough (before the next dose) time-point is the current standard of care. For sirolimus, trough concentrations are only modestly correlated with the area under the curve, with R 2 values ranging from 0.52 to 0.84. Thus, it should not be surprising, even with the use of sirolimus TDM, that patients treated with sirolimus have variable pharmacokinetics, toxicity, and effectiveness. Model-informed precision dosing (MIPD) will be beneficial and should be implemented. The data do not suggest dried blood spots point-of-care sampling of sirolimus concentrations for precision dosing of sirolimus. Future research on precision dosing of sirolimus should focus on pharmacogenomic and pharmacometabolomic tools to predict sirolimus pharmacokinetics and wearables for point-of-care quantitation and MIPD.

Clinical significance of personalized tacrolimus dosing by adjusting to donor CYP3A-status in liver transplant recipients

Donor's CYP3A-status (CYP3A5 genotype and CYP3A4 expression) can provide prognostic information regarding tacrolimus-metabolizing capacity of the liver graft and initial tacrolimus dosing for therapeutic blood concentrations in liver transplants. The present work prospectively investigated whether CYP3A-status guided tacrolimus therapy has any potential clinical benefit for recipients in the early postoperative period.

METHODS

The contribution of preliminary assaying of donor CYP3A-status to the optimization of initial tacrolimus therapy and to the reduction of adverse events (acute rejection, infection, nephrotoxicity) was investigated in 112 liver transplant recipients (CYPtest group) comparing to 101 control patients on tacrolimus concentration guided therapy.

RESULTS

The time for achieving therapeutic tacrolimus concentration was significantly reduced, confirming potential benefit of initial tacrolimus therapy adjusted to donor's CYP3A-status over classical clinical practice of tacrolimus concentration guided treatment (4 vs 8 days, P < 0.0001). Acute rejection episodes (3.6 vs 23.8%, P < 0.0001) and tacrolimus induced nephrotoxicity (8 vs 27%, P = 0.0004) were less frequent in CYPtest group than in control patients, whereas occurrence of infectious disease was not influenced by tacrolimus dosing strategy (3.6 vs 5.9% in CYPtest and control groups, P > 0.05). Acute rejection was often accompanied with tacrolimus blood concentrations lower than 10 ng mL^-1 (20/24 of control and 2/4 of CYPtest patients), while nephrotoxicity was associated with high tacrolimus concentrations (>20 ng mL^-1 ) in the first week after transplantation (13/27 of control and 2/9 of CYPtest patients).

CONCLUSION

CYP3A-status guided therapy significantly improved the risk of misdosing induced early adverse effects (acute rejection, nephrotoxicity).

Clinical Pharmacokinetic and Pharmacodynamic Considerations in the Treatment of Inflammatory Bowel Disease

According to recent clinical consensus, pharmacotherapy of inflammatory bowel disease (IBD) is, or should be, personalized medicine. IBD treatment is complex, with highly different treatment classes and relatively few data on treatment strategy. Although thorough evidence-based international IBD guidelines currently exist, appropriate drug and dose choice remains challenging as many disease (disease type, location of disease, disease activity and course, extraintestinal manifestations, complications) and patient characteristics [(pharmaco-)genetic predisposition, response to previous medications, side-effect profile, necessary onset of response, convenience, concurrent therapy, adherence to (maintenance) therapy] are involved. Detailed pharmacological knowledge of the IBD drug arsenal is essential for choosing the right drug, in the right dose, in the right administration form, at the right time, for each individual patient. In this in-depth review, clinical pharmacodynamic and pharmacokinetic considerations are provided for tailoring treatment with the most common IBD drugs. Development (with consequent prospective validation) of easy-to-use treatment algorithms based on these considerations and new pharmacological data may facilitate optimal and effective IBD treatment, preferably corroborated by effectiveness and safety registries.

Effects of medicines used to treat gastrointestinal diseases on the pharmacokinetics of coadministered drugs: a PEARRL Review

Objectives

Drugs used to treat gastrointestinal diseases (GI drugs) are widely used either as prescription or over-the-counter (OTC) medications and belong to both the 10 most prescribed and 10 most sold OTC medications worldwide. The objective of this review article is to discuss the most frequent interactions between GI and other drugs, including identification of the mechanisms behind these interactions, where possible.

Key findings

Current clinical practice shows that in many cases, these drugs are administered concomitantly with other drug products. Due to their metabolic properties and mechanisms of action, the drugs used to treat gastrointestinal diseases can change the pharmacokinetics of some coadministered drugs. In certain cases, these interactions can lead to failure of treatment or to the occurrence of serious adverse events. The mechanism of interaction depends highly on drug properties and differs among therapeutic categories. Understanding these interactions is essential to providing recommendations for optimal drug therapy.

Summary

Interactions with GI drugs are numerous and can be highly significant clinically in some cases. While alterations in bioavailability due to changes in solubility, dissolution rate, GI transit and metabolic interactions can be (for the most part) easily identified, interactions that are mediated through other mechanisms, such as permeability or microbiota, are less well-understood. Future work should focus on characterising these aspects.

Pharmacokinetics, Pharmacodynamics and Pharmacogenomics of Immunosuppressants in Allogeneic Haematopoietic Cell Transplantation: Part I

Although immunosuppressive treatments and target concentration intervention (TCI) have significantly contributed to the success of allogeneic haematopoietic cell transplantation (alloHCT), there is currently no consensus on the best immunosuppressive strategies. Compared with solid organ transplantation, alloHCT is unique because of the potential for bidirectional reactions (i.e. host-versus-graft and graft-versus-host). Postgraft immunosuppression typically includes a calcineurin inhibitor (cyclosporine or tacrolimus) and a short course of methotrexate after high-dose myeloablative conditioning, or a calcineurin inhibitor and mycophenolate mofetil after reduced-intensity conditioning. There are evolving roles for the antithymyocyte globulins (ATGs) and sirolimus as postgraft immunosuppression. A review of the pharmacokinetics and TCI of the main postgraft immunosuppressants is presented in this two-part review. All immunosuppressants are characterized by large intra- and interindividual pharmacokinetic variability and by narrow therapeutic indices. It is essential to understand immunosuppressants' pharmacokinetic properties and how to use them for individualized treatment incorporating TCI to improve outcomes. TCI, which is mandatory for the calcineurin inhibitors and sirolimus, has become an integral part of postgraft immunosuppression. TCI is usually based on trough concentration monitoring, but other approaches include measurement of the area under the concentration-time curve (AUC) over the dosing interval or limited sampling schedules with maximum a posteriori Bayesian personalization approaches. Interpretation of pharmacodynamic results is hindered by the prevalence of studies enrolling only a small number of patients, variability in the allogeneic graft source and variability in postgraft immunosuppression. Given the curative potential of alloHCT, the pharmacodynamics of these immunosuppressants deserves to be explored in depth. Development of sophisticated systems pharmacology models and improved TCI tools are needed to accurately evaluate patients' exposure to drugs in general and to immunosuppressants in particular. Sequential studies, first without and then with TCI, should be conducted to validate the clinical benefit of TCI in homogenous populations; randomized trials are not feasible, because there are higher-priority research questions in alloHCT. In Part I of this article, we review the alloHCT process to facilitate optimal design of pharmacokinetic and pharmacodynamics studies. We also review the pharmacokinetics and TCI of calcineurin inhibitors and methotrexate.

Over-the-Counter Medications in Cardiac Transplant Recipients: Guidelines for Use

OBJECTIVE:

The purpose of this article is to review the pathophysiology of the denervated heart and the factors that need to be considered before recommending the use of over-the-counter (OTC) medications in the cardiac transplant recipient.

DATA SOURCES:

Pharmacology and therapeutic textbooks, English-language journal articles, and physiology textbooks published between 1969 and 1991.

DATA EXTRACTION:

Case reports, controlled case studies, and textbook chapters evaluating drug interactions with immunosuppressive agents were reviewed. The effects of various OTC medications on the denervated heart were examined and relevant material was extrapolated.

DATA ANALYSIS:

The number of cases or studies in which a particular effect or interaction occurred was reported. Those findings that were less well documented were either identified as such or were not included in the review.

DATA SYNTHESIS:

Common pharmacokinetic and pharmacodynamic interactions with the primary immunosuppressive agents (e.g., cyclosporine, azathioprine, prednisone) are reviewed. The physiology and altered responses of the denervated heart to various medications are also explained. Using this information, recommendations are given for the use and monitoring of OTC analgesics, antacids, laxatives, sleep aids, stimulants, and other medications that may be used in the cardiac transplant recipient.

CONCLUSIONS:

Many OTC medications can be used safely in the cardiac transplant recipient. In each situation, risk/benefit assessments must always be made and therapy should be monitored closely. Most important, patients should always notify the transplant team before adding an OTC product to their immunosuppressive regimen.

The interaction of the diltiazem with oral and intravenous cyclosporine in rats

This study investigated the effect of diltiazem on the bioavailability of oral and intravenous cyclosporine (CsA) in rats. While control rats received normal saline, experimental groups received 60 or 90 mg/kg diltiazem orally for 3 days. Each group divided into 2 equal groups that received a single oral dose or i.v. injection of CsA. Pharmacokinetic parameters were analyzed by nonparametric analysis of variance. Pretreatment with 60 or 90 mg/kg diltiazem decreased the area under the blood CsA concentration-time curve (AUC) of oral CsA compared to control group (54.5% and 65.5% for AUC(0-24), 57.6% and 62.2% for AUC(0-infinity), respectively, p<0.05). Mean CsA maximum concentration (Cmax) decreased from 0.4 +/- 0.1 microg/ml to 0.1 +/- 0.0 microg/mL in rats pretreated with 90 mg/kg diltiazem (p<0.05). The absolute bioavailability after oral administration (F(p.o.)) in the 60 or 90 mg/kg diltiazem groups were lower than the control group (9.6% and 8.5% versus 22.6%). Pretreatment with 90 mg/kg but not 60 mg/kg of diltiazem increased the AUC(0-infinity), elimination half-life (t1/2) of intravenous CsA (116.0%, 219.2%, respectively, p<0.05) and decreased the intravenous CsA clearence (CL(i.v.)) (62.9%, p<0.05). Diltiazem decreased the bioavailability of oral CsA, while it increased the bioavailability of intravenous CsA. One must consider this interaction when administering oral or intravenous CsA concomitantly with diltiazem.

Posttransplant day significantly influences pharmacokinetics of cyclosporine after hematopoietic stem cell transplantation

Cyclosporine-based immunosuppression is common after allogeneic hematopoietic stem cell transplantation (HSCT). Elevated cyclosporine concentrations are associated with significant toxicity and often result in the temporary cessation or discontinuation of cyclosporine. Low blood concentrations also result in significant immunologic risks, primarily graft-versus-host disease and loss of stem cell graft. The pharmacokinetics of cyclosporine are highly complex, and maintaining therapeutic and safe cyclosporine concentrations are challenging. Several clinical factors are known to independently influence in vivo cyclosporine pharmacokinetic behavior. However, in the critically ill patient, several of these clinical factors are generally present simultaneously. Unfortunately, there are no studies that have evaluated the combined effects of these clinical factors on cyclosporine disposition in HSCT. The objective of our study is to determine the population pharmacokinetic parameters of intravenous and oral cyclosporine, evaluate the effects of clinical covariates on cyclosporine pharmacokinetics, and develop a model that estimates clearance (Cl) and dose requirements for an individual HSCT patient with these clinical covariates. The authors analyzed 740 cyclosporine steady-state whole blood concentrations in 129 adult patients obtained between day 0 and discharge or 60 days posttransplant, whichever came first. Patients received intravenous cyclosporine at 2.5 mg/kg every 12 hours if body weight was greater than 50 kg, 2.5 mg/kg every 8 hours if less than 50 kg, or 5 to 7.5 mg/kg/d given as a continuous infusion, beginning on day-3. Patients were converted to oral therapy as tolerated. The influence of clinical covariates on the Cl of cyclosporine was tested with a nonlinear mixed effects model (NONMEM). The tested clinical covariates were age, height, body weight on admission, body surface area, sex, type of hematologic malignancy, transplant type, preparative regimen, baseline serum creatinine, T-cell depletion of graft, number of methotrexate doses, day of onset, and maximum grade of acute graft-versus-host disease. The route and frequency of cyclosporine administration, day posttransplant, total bilirubin level, serum creatinine level, actual body weight, presence of concurrent CYP450 enzyme inhibitors and inducers, or nephrotoxins on the day of the cyclosporine blood measurement were also evaluated. Cyclosporine Cl significantly decreased each week posttransplant. The authors found no significant effect of any of the other tested covariates including total bilirubin on Cl. The final regression model for the estimation of Cl is: Cl (L/hr) = ([body weight in kg - 70] * 0.183 + 22.3) * (day posttransplant factor). The corresponding day posttransplant factor estimates are 1.46, 1.32, 1.20, and 1.0 during days 0 to 7, 8 to 14, 15 to 21 and greater than 21 posttransplant, respectively. The interindividual variability in Cl was 27.7%. The dose of intravenous or oral cyclosporine can be calculated using the estimated Cl. Understanding cyclosporine pharmacokinetics and the clinical events that lead to alterations in Cl and exposure is critical in optimizing immunosuppressive therapy. The authors found that cyclosporine Cl significantly decreased posttransplant until day 21. A pharmacokinetics model was developed that incorporates the day posttransplant to predict cyclosporine Cl. Cyclosporine dose requirements in an individual HSCT patient to achieve the desired therapeutic blood target can be estimated using this model.

Evaluation of a No-Pretreatment Cyclosporin A Assay on the Dade Behring Dimension RxL Clinical Chemistry Analyzer

Background: Monitoring whole-blood concentrations of cyclosporin A (CsA) is common practice in the management of solid organ and bone marrow transplant recipients. In a multicenter study we evaluated a new, direct (no pretreatment) CsA assay on the Dade Behring Dimension RxLTM system and compared results with those from the Abbott TDx CsA immunoassay and a HPLC method.

Methods: Whole-blood samples from heart (n = 111; 35 patients), liver (n = 201; 44 patients), kidney (n = 279; 65 patients), and miscellaneous organ (n = 77; 12 lung, 12 bone marrow, 5 kidney/pancreas, and 1 pancreas patient) recipients were obtained from patient populations of the participating institutions. Routine clinical monitoring of CsA was performed using either the TDx method or HPLC.

Results: The minimum detectable concentration of CsA averaged 9.4 μg/L, and the lower limit of quantification was 30 μg/L. The method was linear from 30 to 500 μg/L. Cross-reactivity with seven different CsA metabolites ranged from 0.0% to 5.7% for the Dimension RxL assay compared with 0.4–15.9% for the TDx assay. Total imprecision (CV) averaged 6.2%, and within-run imprecision averaged 4.9%. Passing–Bablok linear regression analyses of all samples from two sites yielded the following: RxL = 0.81 × TDx − 16.8; and RxL = 1.12 × HPLC − 1.7.

Conclusions: The Dade Behring CsA assay for the random-access Dimension platform offers adequate performance characteristics for routine clinical use, does not require a manual pretreatment step, and demonstrates less cross-reactivity with CsA metabolites than another commonly used immunoassay.

Pharmacokinetic considerations in the treatment of inflammatory bowel disease

This review describes the pharmacokinetics of the major drugs used for the treatment of inflammatory bowel disease. This information can be helpful for the selection of a particular agent and offers guidance for effective and well tolerated regimens. The corticosteroids have a short elimination half-life (t1/2beta) of 1.5 to 4 hours, but their biological half-lives are much longer (12 to 36 hours). Most are moderate or high clearance drugs that are hepatically eliminated, primarily by cytochrome P450 (CYP) 3A4-mediated metabolism. Prednisone and budesonide undergo presystemic elimination. Any disease state or comedication affecting CYP3A4 activity should be taken into account when prescribing corticosteroids. Depending on the preparation used, 10 to 50% of an oral or rectal dose of mesalazine is absorbed. Rapid acetylation in the intestinal wall and liver (t1/2beta 0.5 to 2 hours) and transport probably by P-glycoprotein affect mucosal concentrations of mesalazine, which apparently determine clinical response. Any clinical condition influencing the release and topical availability of mesalazine might modify its therapeutic potential. Metronidazole has high (approximately 90%) oral bioavailability, with hepatic elimination characterised by a t1/2beta of 6 to 10 hours and a total clearance of about 4 L/h/kg. Ciprofloxacin is largely excreted unchanged both renally (about 45% of dose) and extrarenally (25%), with a relatively short t1/2beta (3.5 to 7 hours). Thus, renal function affects the systemic availability of ciprofloxacin. Both mercaptopurine and its prodrug azathioprine are metabolised to active compounds (6-thioguanine nucleotides; 6-TGN) by hypoxanthine-guanine phosphoribosyltransferase and to inactive metabolites by the polymorphically expressed thiopurine S-methyltransferase (TPMT) and xanthine oxidase. Patients with low TPMT activity have a higher risk of developing haemopoietic toxicity. Both mercaptopurine and azathioprine have a short t1/2beta (1 to 2 hours), but the t1/2beta of 6-TGN ranges from 3 to 13 days. Therapeutic response seems to be related to 6-TGN concentration. Almost complete bioavailability has been observed after intramuscular and subcutaneous administration of methotrexate, which is predominantly (85%) excreted as unchanged drug with a t1/2beta of up to 50 hours. Thus, renal function is the major determinant for disposition of methotrexate. Cyclosporin is slowly and incompletely absorbed. It is extensively metabolised by CYP3A4/5 in the liver and intestine (median t1/2beta and clearance 7.9 hours and 0.46 L/h/kg, respectively), and inhibitors and inducers of CYP3A4 can modify response and toxicity. Infliximab is predominantly distributed to the vascular compartment and eliminated with a t1/2beta between 10 and 14 days. No accumulation was observed when it was administered at intervals of 4 or 8 weeks. Methotrexate may reduce the clearance of infliximab from serum.

Old and new drugs used in rheumatoid arthritis: a historical perspective. Part 2: the newer drugs and drug strategies

After a 20-year hiatus, drug development for rheumatoid arthritis resumed in the early 1980s with cyclosporine, continuing in the 1990s with minocycline, leflunomide, and the tumor necrosis factor-alpha antagonists, infliximab and etanercept. Unlike the older disease-modifying antirheumatic drugs (apart from the cytotoxics), the newer drugs were designed with strict reference to proven pathophysiology in rheumatoid arthritis and, apart from minocycline, the intended action of these agents is highly likely the explanation for the observed efficacy. The evidence for the evolution of more rational drug development in rheumatoid arthritis has not altered the fact that efficacy versus toxicity still remains the major determinant in the practical use of these agents, as well as in the use of other, experimental agents briefly discussed. Action, efficacy, and toxicity also determine the rational chronologic use of these drugs alone and, in particular, in combination.

Anaesthesia for patients with a transplanted organ

Increasing numbers of individuals leading normal lives have transplanted organs. They may appear in any hospital for treatment of trauma or general diseases. Common anaesthesia methods can be used for these patients, but safe conduct of anaesthesia requires knowledge of the immunosuppression, risk factors, and altered physiology or drug actions. This article reviews the anaesthesia-related literature on patients with transplanted organs.

Neoral use in the renal transplant recipient

This review and critical analysis of current trends of immunosuppression management in pediatric transplantation provides evidence and support for the continued role of Neoral as an indispensable part of immunosuppressive protocols. CyA formulation influences clinical outcomes such as acute rejection, as confirmed by several multicenter studies. The CyA microemulsion formulation reduces pharmacokinetic variability and its consequent impact on outcomes over the long term. An advanced TDM strategy can improve the effectiveness and safety of immunosuppression in both de novo and maintenance renal transplant patients. There are potential risks resulting from CyA withdrawal strategies. Neoral is an indispensable part of combination protocols in renal transplantation.

Neoral use in the liver transplant recipient

This review and critical analysis of current trends of immunosuppression management in liver transplantation provides evidence and support for the continued role of Neoral as an indispensable part of immunosuppressive protocols. CyA formulation influences clinical outcomes such as acute rejection; this is confirmed by several multicenter studies. The CyA microemulsion formulation provides more reliable and effective absorption characteristics, and it may provide more rapid and complete immunosuppression in the de novo patient than IV administration. An advanced TDM strategy, particularly a 2-hour post-dose blood level (C-2), can improve the effectiveness and safety of immunosuppression in de novo liver transplant patients. There are potential risks resulting from CyA withdrawal strategies, as there is no evidence supporting this strategy. Neoral is an indispensable part of combination protocols in liver transplantation.

Macrolide drug interactions: an update

OBJECTIVE

To describe the current drug interaction profiles for the commonly used macrolides in the US and Europe, and to comment on the clinical impact of these interactions.

DATA SOURCES

A MEDLINE search (1975-1998) was performed to identify all pertinent studies, review articles, and case reports. When appropriate information was not available in the literature, data were obtained from the product manufacturers.

STUDY SELECTION

All available data were reviewed to provide an unbiased account of possible drug interactions.

DATA EXTRACTION

Data for some of the interactions were not available from the literature, but were available from abstracts or company-supplied materials. Although the data were not always explicit, the best attempt was made to deliver pertinent information that clinical practitioners would need to formulate practice opinions. When more in-depth information was supplied in the form of a review or study report, a thorough explanation of pertinent methodology was supplied.

DATA SYNTHESIS

Several clinically significant drug interactions have been identified since the approval of erythromycin. These interactions usually were related to the inhibition of the cytochrome P450 enzyme systems, which are responsible for the metabolism of many drugs. The decreased metabolism by the macrolides has in some instances resulted in potentially severe adverse events. The development and marketing of newer macrolides are hoped to improve the drug interaction profile associated with this class. However, this has produced variable success. Some of the newer macrolides demonstrated an interaction profile similar to that of erythromycin; others have improved profiles. The most success in avoiding drug interactions related to the inhibition of cytochrome P450 has been through the development of the azalide subclass, of which azithromycin is the first and only to be marketed. Azithromycin has not been demonstrated to inhibit the cytochrome P450 system in studies using a human liver microsome model, and to date has produced none of the classic drug interactions characteristic of the macrolides.

CONCLUSIONS

Most of the available data regarding macrolide drug interactions are from studies in healthy volunteers and case reports. These data suggest that clarithromycin appears to have an interaction profile similar to that of erythromycin. Given this similarity, it is important to consider the interaction profile of clarithromycin when using erythromycin. This is especially necessary as funds for further studies of a medication available in generic form (e.g., erythromycin) are limited. Azithromycin has produced few clinically significant interactions with any agent cleared through the cytochrome P450 enzyme system. Although the available data are promising, the final test should come from studies conducted in patients who are taking potentially interacting compounds on a chronic basis.

Effects of cyclosporin on the pharmacokinetics of propranolol after intravenous and oral administration to control rats and to rats with uranyl nitrate-induced acute renal failure

The effects of cyclosporin on the pharmacokinetics of propranolol have been investigated after intravenous and oral administration of the drugs to control rats and to rats with uranyl nitrate-induced acute renal failure. The effects of intravenous cyclosporin, 30 mg kg(-1), on the pharmacokinetics of intravenous propranolol, 3 mg kg(-1), were significant both in control rats and in rats with uranyl nitrate-induced acute renal failure; after intravenous administration of cyclosporin plasma concentrations of propranolol were significantly lower, the area under the plasma concentration-time curve (AUC) for propranolol from time zero to time infinity was significantly smaller, and the time-averaged total body clearance of propranolol was significantly faster. The effects of oral cyclosporin, 100 mg kg(-1), on the pharmacokinetics of oral propranolol, 10 mg kg(-1), were also significant, both in control rats and in rats with uranyl nitrate-induced acute renal failure; after administration of oral cyclosporin plasma concentrations of propranolol were significantly higher and the AUC of propranolol was significantly greater. These data suggest that cyclosporin increases the elimination of propranolol, and that the first-pass effects of propranolol are reduced, or gastrointestinal absorption of propranolol is increased, or both, by cyclosporin.

Cyclosporine suppresses rat hepatic cytochrome P450 in a time-dependent manner

BACKGROUND

Cyclosporine is a potent immunosuppressant know to selectively suppress specific cytochrome P450 (P450) isoforms following chronic therapy in the rat. Cyclosporine undergoes significant hepatic metabolism in the rat, primarily due to P450 3A isoforms. Hence, alterations in hepatic metabolism of cyclosporine may lead to changes in drug pharmacokinetics or pharmacodynamics. The purpose of this study was to examine the temporal effect of chronic cyclosporine dosing on P450 protein expression and metabolic activity in a rat model of chronic cyclosporine nephropathy.

METHODS

Adult male rats were administered cyclosporine 15 mg/kg/day or vehicle 1 ml/kg/day by subcutaneous injection for up to 28 days. To examine whether or not metabolic activity recovered following drug removal, additional rats were administered cyclosporine for 28 days followed by vehicle for up to an additional 15 days. Hepatic P450 protein expression and microsomal metabolic activity were measured by Western blot analysis and in vitro steroid hydroxylation, respectively.

RESULTS

Cyclosporine trough levels progressively increased over the 28 days period and were still measurable for up to 15 days after discontinuation. Immunoblot analysis indicated that chronic cyclosporine treatment suppressed P450 3A2 expression and in vitro steroid hydroxylation in a time-dependent manner. Fifteen days following discontinuation of cyclosporine dosing, hepatic metabolic activity and microsomal P450 3A2 levels returned to near pre-dosing levels.

CONCLUSIONS

We conclude that the time-dependent P450 suppression by cyclosporine may at least partially explain the variability in cyclosporine pharmacokinetics. These studies support the hypothesis that hepatic isoforms other than P450 3A2 may be responsible for cyclosporine metabolism during chronic treatment in the rat.

Phase I safety and pharmacokinetic studies of brequinar sodium after single ascending oral doses in stable renal, hepatic, and cardiac allograft recipients

Brequinar sodium (BQR), a substituted 4-quinoline carboxylic acid, was in clinical development in combination with cyclosporine (CsA) as a potentially effective therapy for the treatment and prophylaxis of rejection in organ transplant patients. This phase I study was performed in stable renal, hepatic, and cardiac transplant patients receiving CsA and prednisone maintenance therapy for immunosuppression. The pharmacokinetic objectives of this study were to characterize the pharmacokinetics of (a) single oral 0.5- to 4-mg/kg doses of BQR when given in combination with CsA and prednisone to stable renal, hepatic, and cardiac transplant patients and (b) steady-state oral doses of CsA, with and without single oral doses of BQR. In all three patient populations, the pharmacokinetics of BQR were characterized by a lower oral clearance (12-19 mL/min) than that seen in previous studies in patients with cancer (approximately 30 mL/min at similar doses) and a long terminal half life (13-18 hrs). This slower oral clearance for BQR could be due either to a drug interaction between BQR and CsA or to altered clearance or metabolic processes in patients with transplants. Steady-state CsA trough levels and the oral clearance of CsA were not affected by BQR coadministration. Among the three transplant populations, the cardiac transplant patients had lower oral clearance values of BQR and of CsA. The cause of this lower clearance is not known. Safety results indicate that BQR was well tolerated by this patient population.

Whole blood cyclosporin monitoring in liver and heart transplant patients: evaluation of the specificity of a fluorescence polarization immunoassay and an enzyme-multiplied immunoassay technique

The specificity of two cyclosporin immunoassays were evaluated. Eleven patients were followed for the first four weeks after heart (n = 3) or liver (n = 8) transplantation. Cyclosporin A (CsA) monitoring was performed concomitantly by a monoclonal fluorescence polarization immunoassay (mFPIA) and enzyme-multiplied immunoassay technique (EMIT) during this period. For several patients, cyclosporin monitoring was also performed by high performance liquid chromatography (HPLC) or by polyclonal fluorescence polarization immunoassay (pFPIA). Liver function was assessed by follow-up of plasma total bilirubin, gamma-glutamyl transferase and alkaline phosphatase and renal function by plasma creatinine. All the patients presented episodes of impaired liver function. Higher CsA levels were found using mFPIA measurements as compared to the EMIT measurements (ratio mFPIA:EMIT (medium range) = 1.4 (1.0-2.3)). A higher degree of cross-reactivity of the antibody used in the mFPIA as compared to the EMIT was demonstrated by specific measurements of CsA and its primary metabolite, AM1, by HPLC.

Seizures after orthotopic liver transplantation

Orthotopic liver transplantation is the accepted treatment for endstage liver disease. In the US alone, more than 3000 patients receive liver transplants yearly distributed through more than 100 liver transplant programmes. Neurological complications occur in up to 47% of these patients. Among them, seizures are one of the most common. They tend to occur during the first few weeks after transplantation. Generalized seizures are the most frequently encountered. Their aetiology is usually multifactorial requiring a comprehensive diagnostic and therapeutic approach. Seizures must be differentiated from a variety of behavioural and movement disorders. In this review article, the frequency and time of occurrence of seizures, their types and aetiology, diagnostic approaches and treatment are discussed.

Anaesthesia for patients after lung transplantation

PURPOSE

The purpose of this article is to review the literature on post lung transplant patients presenting for surgery and anaesthesia and to provide insight into their perioperative management.

SOURCE

Articles and books were identified via a Medline search and through a review of the bibliographies of these sources.

PRINCIPLE FINDINGS

Single and double lung transplantation is becoming more common and the period of survival is increasing. As a result, more of these patients are presenting for surgery and anaesthesia. Also, it is increasingly likely that these patients may present, either for emergency or elective surgery, to anaesthetists with limited experience in this field. These patients have considerable medical, physiological and pharmacological problems which need to be understood.

CONCLUSION

Anaesthesia, local, regional, or general, can be safely delivered to these patients provided that the physiology and pathophysiology of the transplanted lung, the pharmacology of the immunosuppressive agents, and the underlying surgical condition are understood.

Pharmacology of immunosuppressive medications used in renal diseases and transplantation

As understanding of the molecular basis for the immune response has expanded rapidly, so have the possibilities for designing therapeutic interventions that are more effective, more specific, and safer than current treatment options. The promise of therapeutic advances in the future is based on the rapidly expanding insights into the pathogenesis of abnormal immunologic reactions. Nowhere is the understanding of molecular mechanisms, pathophysiology, and targeted therapy more relevant than in the field of renal transplantation, which makes up much of the clinical database for the use of immunosuppressive therapy for renal disease. Despite the recent advances in basic immunology, clinical validation of new agents and approaches is lacking for most drugs at present. This review will focus in the pharmacology of agents used in the therapy of immunologic renal disease and in renal transplantation. It should be recognized that clinical pharmacology and experience with newer agents is limited, and potential utility is based largely on experimental data.

Cyclosporine monitoring in allogeneic bone marrow transplantation

Objective. To review the benefits and limitations of cyclosporine concentration monitoring to maximize efficacy of graft-versus-host disease prophylaxis and minimize risk of cyclosporine toxicity.

Data Sources. A CANCERLIT search of articles published from 1980 to 1995, using the MESH head ings "cyclosporine," and "bone marrow transplant." References listed in the identified publications were reviewed for additional pertinent literature.

Study Selection. Human clinical trials and re view articles evaluating cyclosporine pharmacokinet ics and pharmacodynamics in patients status follow ing bone marrow transplantation.

Data synthesis. Cyclosporine concentration monitoring has been proposed due to the critical need for drug efficacy, substanial drug-induced toxic ity, and high pharmacokinetic variability. Selection of biological fluid (blood, serum, or plasma) for moni toring as well as the selectivity of the assay employed for cyclosporine versus its metabolites, is a critical factor in evaluating this literature. Both the relation ship between cyclosporine trough concentration and clinical outcome has been evaluated in this patient population.

Conclusions. Cyclosporine trough concentra tion monitoring, especially in the blood using an assay specific for the parent compound, has a role in the care of the allogeneic bone marrow transplantation patient. Cyclosporine levels have been correlated with risk for nephrotoxicity and to a lesser extent hepatotoxicity. Low cyclosporine levels are one fac tor contributing to the risk for acute graft-versus-host disease in these patients. Proper evaluation of cyclo sporine levels with due consideration to patient spe cific factors can assist the clinician in maximizing the chances for graft-versus-host disease prevention while reducing the risk of cyclosporine side effects.

Clinically significant drug interactions with cyclosporin. An update

Since its approval in 1983 for immunosuppressive therapy in patients undergoing organ and bone marrow transplants, cyclosporin has had a major impact on organ transplantation. It has significantly improved 1-year and 2-year graft survival rates, and decreased morbidity in kidney, liver, heart, heart-lung and pancreas transplantation. Several studies have supported the efficacy of cyclosporin in preventing graft-versus-host disease in bone marrow transplantation. Cyclosporin is also possibly effective in treating diseases of autoimmune origin and as an antineoplastic agent. The introduction of therapeutic drug monitoring of cyclosporin was extremely useful because of the wide inter- and intraindividual variability in the pharmacokinetics of cyclosporin after oral or intravenous administration. Optimal long term use of cyclosporin requires careful monitoring of the blood (or plasma) concentrations. Sustained and clinically significant drug-drug interactions can occur during long term therapy with cyclosporin. The coadministration of multiple drugs with cyclosporin could result in graft rejection, renal dysfunction or other undesirable effects. Any interaction that leads to modified cyclosporin concentrations is of potential clinical importance. Cyclosporin itself may have significant effects on the pharmacokinetics and/or pharmacodynamics of coadministered drugs, such as digoxin, HMG-CoA reductase inhibitors and antineoplastic drugs affected by multidrug resistance. Many drugs have been shown to affect the pharmacokinetics and/or pharmacodynamics of cyclosporin. Interactions between cyclosporin and danazol, diltiazem, erythromycin, fluconazole, itraconazole, ketoconazole, metoclopramide, nicardipine, verapamil, carbamazepine, phenobarbital (phenobarbitone), phenytoin, rifampicin (rifampin) and cotrimoxazole (trimethoprim/sulfamethoxazole) are well documented in a large number of patients. Other interactions (such as those with aciclovir, estradiol and imipenem) are documented only in isolated case studies.

The emerging role of cytochrome P450 3A in psychopharmacology

Recent advances in molecular pharmacology have allowed the characterization of the specific isoforms that mediate the metabolism of various medications. This information can be integrated with older clinical observations to begin to develop specific mechanistic and predictive models of psychotropic drug interactions. The polymorphic cytochrome P450 2D6 has gained much attention, because competition for this isoform is responsible for serotonin reuptake inhibitor-induced increases in tricyclic antidepressant concentrations in plasma. However, the cytochrome P450 3A subfamily and the 3A3 and 3A4 isoforms (CYP3A3/4) in particular are becoming increasingly important in psychopharmacology as a result of their central involvement in the metabolism of a wide range of steroids and medications, including antidepressants, benzodiazepines, calcium channel blockers, and carbamazepine. The inhibition of CYP3A3/4 by medications such as certain newer antidepressants, calcium channel blockers, and antibiotics can increase the concentrations of CYP3A3/4 substrates, yielding toxicity. The induction of CYP3A3/4 by medications such as carbamazepine can decrease the concentrations of CYP3A3/4 substrates, yielding inefficiency. Thus, knowledge of the substrates, inhibitors, and inducers of CYP3A3/ and other cytochrome P450 isoforms may help clinicians to anticipate and avoid pharmacokinetic drug interactions and improve rational prescribing practices.

Innovative treatment approaches for rheumatoid arthritis. Cyclosporin, leflunomide and nitrogen mustard

Cyclosporin A (CSA) Cyclosporin inhibits IL-2 release and T-cell activation and, secondarily, affects B-cell function. It also inhibits bone resorption, at least in vitro. This drug's bio-availability averages 25-35% but is highly variable. Food and grapefruit juice enhance bio-availability and newer formulations may make its absorption more reliable. It is highly concentrated in fatty tissues and red blood cells but does not cross the blood-brain barrier. CSA is metabolized to numerous metabolites by the liver and its elimination half-life is 6-12 hours in the absence of severe liver disease. Biliary excretion accounts for 94% of CSAs elimination. Because it is highly metabolized, its metabolism can be inhibited by other drugs (e.g. ketoconazole and erythromycin) or its metabolism can be induced (e.g. anticonvulsants). Cyclosporin is more effective than placebo for the treatment of rheumatoid arthritis and as effective as other antirheumatics. There is potential for the use of CSAs in DMARD combinations. The principal toxicities of cyclosporin are gastro-intestinal and renal, with the latter being of more concern. Leflunomide (LF). Leflunomide may be a pyrimidine synthesis inhibitor, although tyrosine kinase inhibitor may also be part of its mechanism of action. Its active metabolite is excreted renally to a large degree, with a prolonged elimination half-life of about 11 days. Since LF is activated by liver metabolism, renal failure may have less effect on kinetics than severe liver disease. Early data on efficacy indicate efficacy at 10-25 mg/day, although more well-controlled data is necessary. Toxicity relates to the skin, liver and GI tract, although some degree of weight loss was also found. Nitrogen mustard (NM). Nitrogen mustard is an alkylating agent whose pharmacokinetics are poorly understood. Small, open studies in RA indicate that NM has a potential for relatively rapid response (1-2 weeks) but, clearly, much work remains to be done. As an alkylating agent, GI and hematological toxicities are of greatest concern.

Macrolides versus azalides: a drug interaction update

OBJECTIVE

To describe the current drug interaction profiles for all approved and investigational macrolide and azalide antimicrobials, and to comment on the clinical impact of these interactions when appropriate.

DATA SOURCES

MEDLINE was searched to identify all pertinent studies, review articles, and case reports from 1975 to 1995. When appropriate information was not available in the literature, data were obtained from the product manufacturers.

STUDY SELECTION

All available data were reviewed to give an unbiased account of possible drug interactions.

DATA EXTRACTION

Data for some of the interactions were not available from the literature, but were available from abstracts or from company-supplied materials. Although the data were not always entirely explicative, the best attempt was made to deliver the pertinent information that clinical practitioners would need to formulate practice opinions. When more in-depth information was supplied in the form of a review or study report, a thorough explanation of pertinent methodology was supplied.

DATA SYNTHESIS

Since the introduction of erythromycin into clinical practice, there have been several clinically significant drug interactions identified throughout the literature associated with this drug. These interactions have been caused mostly by inhibition of the CYP3A subclass of hepatic enzymes, thereby decreasing the metabolism of any other agent given concurrently that is also cleared through this mechanism. With the development and marketing of several new macrolides, it was hoped that the drug interaction profile associated with this class would improve. This has been met with variable success. Although some of the extensions of the 14-membered ring macrolides have shown an incidence of interactions equal to that of erythromycin, others have shown improved profiles. In contrast, the 16-membered ring macrolides have demonstrated a much improved, though not absent, interaction profile. The most success in avoiding drug interactions through structure modification has been accomplished with the development of the azalide class, of which azithromycin is the first to be approved for marketing. This agent has to date produced none of the classic drug interactions that most macrolides have demonstrated in patient care.

CONCLUSIONS

The introduction of new 14- and 16-membered ring macrolides appears to have had a variable effect in modifying the incidence of drug interactions associated with this class. Azithromycin, a member of the new azalide class, has to date produced fewer clinically significant interactions than other azalides with any agent that is cleared through the CYP3A system.(ABSTRACT TRUNCATED AT 250 WORDS)

Macrolide antibacterials. Drug interactions of clinical significance

Macrolide antibiotics can interact adversely with commonly used drugs, usually by altering metabolism due to complex formation and inhibition of cytochrome P-450 IIIA4 (CYP3A4) in the liver and enterocytes. In addition, pharmacokinetic drug interactions with macrolides can result from their antibiotic effect on microorganisms of the enteric flora, and through enhanced gastric emptying due to a motilin-like effect. Macrolides may be classified into 3 different groups according to their affinity for CYP3A4, and thus their propensity to cause pharmacokinetic drug interactions. Troleandomycin, erythromycin and its prodrugs decrease drug metabolism and may produce drug interactions (group 1). Others, including clarithromycin, flurithromycin, midecamycin, midecamycin acetate (miocamycin; ponsinomycin), josamycin and roxithromycin (group 2) rarely cause interactions. Azithromycin, dirithromycin, rikamycin and spiramycin (group 3) do not inactivate CYP3A4 and do not engender these adverse effects. Drug interactions with carbamazepine, cyclosporin, terfenadine, astemizole and theophylline represent the most frequently encountered interactions with macrolide antibiotics. If the combination of a macrolide and one of these compounds cannot be avoided, serum concentrations of concurrently administered drugs should be monitored and patients observed for signs of toxicity. Rare interactions and those of dubious clinical importance are those with alfentanil and sufentanil, antacids and cimetidine, oral anticoagulants, bromocriptine, clozapine, oral contraceptive steroids, digoxin, disopyramide, ergot alkaloids, felodipine, glibenclamide (glyburide), levodopa/carbidopa, lovastatin, methylprednisolone, phenazone (antipyrine), phenytoin, rifabutin and rifampicin (rifampin), triazolam and midazolam, valproic acid (sodium valproate) and zidovudine.

Optimizing the use of cyclosporine in renal transplantation

OBJECTIVE

To review the existing data on the use of cyclosporine (CsA) in kidney transplantation, particularly with respect to therapeutic drug monitoring.

DATA SOURCES

A literature search was conducted of applicable articles related to therapeutic drug monitoring of cyclosporine in renal transplantation. Previous consensus guidelines were examined. Discussions on issues related to this topic convened in Toronto, ON, on June 15-16, 1994.

DATA SYNTHESIS

The literature was analyzed to examine patient factors and drug interactions affecting CsA concentrations, the effect of CsA concentrations on patient outcome, current methods of analysis, pharmacodynamic monitoring, and new immunosuppressants.

CONCLUSIONS

CsA has improved the success of kidney transplantation, reducing the incidence and severity of acute rejection and improving short-term patient and graft survival. The rate of graft loss after the first year (primarily due to chronic rejection) has remained largely unchanged. Sandimmune Neoral offers promise due to its better bioavailability and limited dependence on bile flow for absorption. Long-term studies are underway to determine its effectiveness and safety. Indications for therapeutic drug monitoring for CsA are provided.

The effect of grapefruit juice on cyclosporine and prednisone metabolism in transplant patients

OBJECTIVE

To estimate the effect of grapefruit juice on cyclosporine and prednisone metabolism.

METHODS

This was an open, placebo-controlled, two-way crossover study performed in the academic departments of clinical pharmacology and nephrology. On two study occasions, 12 kidney transplant patients with stable cyclosporine trough levels received either grapefruit juice or water every 3 hours for a period of 30 hours. The main outcome measures were peak concentration and time to peak, area under the concentration-time curve, the ratio of the area under the curve of the metabolites/area under the curve of the parent drug, terminal half-life, and 24-hour trough levels of cyclosporine.

RESULTS

Grapefruit juice increased the peak concentration of cyclosporine by 185 ng/ml (95% confidence interval, 60 to 310; p = 0.008). The ratio of the area under the curve of the metabolites of cyclosporine to the area under the curve of cyclosporine was reduced by 0.137 on the grapefruit day (95% confidence interval, -0.221 to -0.054; p = 0.004). After grapefruit juice, no significant changes were observed in the area under the curve and the time to peak of cyclosporine, prednisone, and prednisolone. Cyclosporine trough levels were unchanged by grapefruit juice.

CONCLUSIONS

Grapefruit juice inhibits the metabolism of cyclosporine for a brief period after administration, which may be explained by the inhibition of cytochrome P450 enzymes in the gut wall and to a lesser extent by inhibition of these enzymes in the liver. Grapefruit juice can be one of the factors leading to intraindividual variability in the pharmacokinetics of cyclosporine. Grapefruit juice had no significant effect on the metabolism of prednisone or prednisolone.

Calcium-channel entry blocker therapy for hypertensive patients with concomitant renal impairment: a focus on isradipine

In the treatment of hypertension in renally impaired patients, normalization of blood pressure alone may not be sufficient to prevent significant morbidity to the kidneys. Treatment must reduce pressure in the renal vasculature, otherwise glomerular filtration rate and renal plasma flow will continue to deteriorate. Isradipine a dihydropyridine calcium-channel blocker, has been investigated as a suitable treatment in this setting. Isradipine maintains glomerular filtration rate, preserves or enhances renal plasma flow, decreases renal vascular resistance, maintains or reduces filtration fraction, and exerts a sustained natriuretic effect, all of which may enable isradipine to slow the rate of progression of renal deterioration. In addition, isradipine may decrease proteinuria and may decrease glomerular capillary pressure by dilating both the efferent and afferent arterioles. Unlike older calcium-channel blockers, isradipine exhibits minimal cardiodepressant activity and is not associated with any negative inotropic effects. It is metabolized in the liver and dosage adjustments may not be necessary when administered to patients with renal insufficiency. Isradipine has a favorable renal effect profile and also has several properties that meet the requirements of other patient populations where an extra measure of antihypertensive safety is required, such as diabetics, dialysis patients, and transplant recipients. Side effects with isradipine are usually mild and transient, occurring in a dose-dependent manner.

A review of assay methods for cyclosporin. Clinical implications

Cyclosporin is a unique immunosuppressive agent with a narrow therapeutic range. The pharmacokinetics of the drug present substantial within- and between-patient variability and drug interactions can significantly alter blood cyclosporin concentrations. Monitoring of cyclosporin concentrations in blood is an invaluable and essential aid in adjusting dosage to ensure adequate immunosuppression while minimising toxicity. The principal rationale behind therapeutic monitoring of cyclosporin is the fact that the incidence of rejection is higher at low cyclosporin concentrations and toxicity occurs more often at high concentrations. In renal transplant recipients, cyclosporin concentrations help to discriminate between insufficient immunosuppression and cyclosporin-induced nephrotoxicity. There are several methods available, both specific and nonspecific, for the routine measurement of cyclosporin. Radioimmunoassay and fluorescence polarisation immunoassay are most widely employed, while high performance liquid chromatography remains the reference procedure. The allegedly specific immunoassays tend to slightly overestimate the actual blood cyclosporin concentrations. There is a need for assay systems capable of measuring the biological activity of cyclosporin. Cyclosporin concentrations should be determined by a specific method, using whole blood as the sample matrix. The routine monitoring of individual cyclosporin metabolites is not warranted, but characterising the metabolite pattern of cyclosporin by concomitant use of a nonspecific and a specific assay can be clinically useful in patients with cyclosporin-associated toxicity or impaired liver function. In organ transplantation, measurement of blood cyclosporin concentration should be continued periodically as long as the therapy continues, whereas monitoring is only indicated in special circumstances in patients with autoimmune and other nontransplant diseases. The assessment of a 'therapeutic window' for cyclosporin is complicated for several reasons and definite target ranges cannot be given. Cyclosporin concentrations should always be interpreted in conjunction with the recent blood concentration history and other relevant clinical and laboratory data.

Pharmacokinetic drug interactions of macrolides

The macrolide antibiotics include natural members, prodrugs and semisynthetic derivatives. These drugs are indicated in a variety of infections and are often combined with other drug therapies, thus creating the potential for pharmacokinetic interactions. Macrolides can both inhibit drug metabolism in the liver by complex formation and inactivation of microsomal drug oxidising enzymes and also interfere with microorganisms of the enteric flora through their antibiotic effects. Over the past 20 years, a number of reports have incriminated macrolides as a potential source of clinically severe drug interactions. However, differences have been found between the various macrolides in this regard and not all macrolides are responsible for drug interactions. With the recent advent of many semisynthetic macrolide antibiotics it is now evident that they may be classified into 3 different groups in causing drug interactions. The first group (e.g. troleandomycin, erythromycins) are those prone to forming nitrosoalkanes and the consequent formation of inactive cytochrome P450-metabolite complexes. The second group (e.g. josamycin, flurithromycin, roxithromycin, clarithromycin, miocamycin and midecamycin) form complexes to a lesser extent and rarely produce drug interactions. The last group (e.g. spiramycin, rokitamycin, dirithromycin and azithromycin) do not inactivate cytochrome P450 and are unable to modify the pharmacokinetics of other compounds. It appears that 2 structural factors are important for a macrolide antibiotic to lead to the induction of cytochrome P450 and the formation in vivo or in vitro of an inhibitory cytochrome P450-iron-nitrosoalkane metabolite complex: the presence in the macrolide molecules of a non-hindered readily accessible N-dimethylamino group and the hydrophobic character of the drug. Troleandomycin ranks first as a potent inhibitor of microsomal liver enzymes, causing a significant decrease of the metabolism of methylprednisolone, theophylline, carbamazepine, phenazone (antipyrine) and triazolam. Troleandomycin can cause ergotism in patients receiving ergot alkaloids and cholestatic jaundice in those taking oral contraceptives. Erythromycin and its different prodrugs appear to be less potent inhibitors of drug metabolism. Case reports and controlled studies have, however, shown that erythromycins may interact with theophylline, carbamazepine, methylprednisolone, warfarin, cyclosporin, triazolam, midazolam, alfentanil, disopyramide and bromocriptine, decreasing drug clearance. The bioavailability of digoxin appears also to be increased by erythromycin in patients excreting high amounts of reduced digoxin metabolites, probably due to destruction of enteric flora responsible for the formation of these compounds. These incriminated macrolide antibiotics should not be administered concomitantly with other drugs known to be affected metabolically by them, or at the very least, combined administration should be carried out only with careful patient monitoring.(ABSTRACT TRUNCATED AT 400 WORDS)

Omeprazole drug interaction studies

This review examines the literature on drug interactions with omeprazole. Different mechanisms have been proposed as potential causes for such interactions. First, the absorption of some drugs might be altered due to the decreased intragastric acidity resulting from omeprazole treatment. There was no effect of omeprazole on the absorption of amoxycillin, bacampicillin and alcohol, while the amount of digoxin and nifedipine absorbed was increased by 10 and 21%, respectively, both increases probably being of no clinical significance. Secondly, the metabolism of high clearance drugs might be altered by changes in liver blood flow, although that is not affected by omeprazole, as indicated by the unchanged elimination of indocyanine green. In addition, the clearance of intravenously administered lidocaine (lignocaine) [a high clearance drug] was unaffected by omeprazole, further indicating that the latter does not alter liver blood flow. Thirdly, since omeprazole is a substituted benzimidazole, it might have the potential to interfere with the metabolism of other drugs by altering the activity of drug metabolising enzymes in the cytochrome P450 system, through either induction or inhibition. There is no indication of induction of this enzyme system in any interaction study with omeprazole. As regards inhibition, on the other hand, there is now considerable information available which indicates that omeprazole has the potential to partly inhibit the metabolism of drugs metabolised to a great extent by the cytochrome P450 enzyme subfamily IIC (diazepam, phenytoin), but not of those metabolised by subfamilies IA (caffeine, theophylline), IID (metoprolol, propranolol) and IIIA (cyclosporin, lidocaine, quinidine). Since relatively few drugs are metabolised mainly by IIC compared with IID and IIIA, the potential for omeprazole to interfere with the metabolism of other drugs appears to be limited.